For me, it seems it's with every mod, whether it is good or bad. I'm not sure what to do! It seems others have similar issues, but usually it is due to a bad mod. I even tried basic test mods, such as the No Intro mod, and the same problem would occur. I tried moving my Sims 4 files to my desktop, to start a new game, and it worked fine, but only because it was fresh files with 0 mods. The only way to get my game to load was to have no mods at all. I figured it was one or two mods, so I took all of them out, and it loaded fine. However, the moment I installed mods, my game wouldn't load and would display an error message. I learned how to install mods, and followed all the instructions thoroughly, being sure they were up to date with my version of sims (which is the latest version, effective March 20th, 2018). At this time, the boxes are still ticked, even after doing the desktop method. I also made sure to restart it, so the changes were in effect. I made sure to check the right tick boxes, such as allowing custom content and script mods in the options menu on my game. The game loads completely fine, and I had fun, but then decided to start downloading mods. I made sure to play the game first, to get all the files in my computer.
I was excited to dive into using custom content and mods with my game. I have recently installed the Sims 4 for the first time, using Origin.